Negative electrode active material for lithium secondary battery and method of preparing the same

ABSTRACT

The present specification relates to a negative electrode active material which includes a silicon-based composite represented by SiO a  (0≤a&lt;1), and a carbon coating layer distributed on a surface of the silicon-based composite, and which has a bimodal pore structure including nanopores and mesopores. In a lithium secondary battery including the negative electrode active material, an oxygen content in the silicon-based composite can be controlled to improve initial efficiency and capacity characteristics, and a specific surface area can also be controlled, and thus a side reaction with electrolyte can be reduced.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2015-0135474 filed on Sep. 24, 2015, the disclosureof which is incorporated herein by reference in their entirety.

Technical Field

The present specification relates to a negative electrode activematerial for a lithium secondary battery which includes silicon-basedcomposite, and a method of preparing the same.

Background Art

Lithium secondary batteries, which have been recently spotlighted as apower source of portable and small electronic devices, may exhibit highdischarge voltages that are two times or more than those of batteriesusing a typical alkaline aqueous solution by using an organicelectrolyte solution, and thus exhibit high energy density.

Oxides, formed of lithium and a transition metal, which have a structurecapable of intercalating lithium such as LiCoO₂, LiMn₂O₄,LiNi_(i-x)Co_(x)O₂ (0<x<1) and the like have been mainly used as apositive electrode active material of a lithium secondary battery, andvarious types of carbon-based materials including artificial graphite,natural graphite, and hard carbon, which are capable of intercalatingand deintercalating lithium, have been used as a negative electrodeactive material.

Graphite is mainly used as a negative electrode material of the lithiumsecondary battery. However, graphite has a low capacity per unit mass of372 mAh/g and it may be difficult to prepare a high-capacity lithiumsecondary battery using graphite.

As a negative electrode material exhibiting a higher capacity thangraphite, a material forming an intermetallic compound with lithium,such as silicon, tin, and an oxide thereof, is promising. However, thereis a problem in that volumes of the above-described materials expandbecause crystal structures thereof are changed when absorbing andstoring lithium. When silicon absorbs and stores the maximum amount oflithium, the silicon is transformed into Li_(4.4)Si and the volumethereof expands due to charging. In this case, as a rate of increase involume by charging, the volume may expand up to about 4.12 times thevolume of the silicon before the volume expansion.

Therefore, many studies on an increase in the capacity of a negativeelectrode material such as silicon have been conducted. However, since ametal such as silicon (Si), tin (Sn), aluminum (Al) or the like isreacted with lithium during charging and discharging, volume expansionand contraction occur, and thus cycle characteristics of the battery aredegraded.

DISCLOSURE Technical Problem

The present specification has been made in view of the above, and anobject of the present specification is to provide a negative electrodeactive material for a lithium secondary battery capable of improving theinitial efficiency and lifetime characteristics of the lithium secondarybattery and preventing a side reaction with an electrolyte bycontrolling a specific surface area, and a preparation method thereof,and specifically, to provide a negative electrode active materialincluding a silicon-based composite, eventually a silicon with a porestructure in consideration of both volume expansion and the capacityimprovement.

Technical Solution

An embodiment of the present specification provides a negative electrodeactive material which includes a silicon-based composite represented bySiO_(a) (0≤a<1), and a carbon coating layer distributed on a surface ofthe silicon-based composite, wherein the silicon-based composite has abimodal pore structure including mesopores and macropores.

According to another embodiment of the present specification, thesilicon-based composite may have the bimodal pore structure formedentirely from an inner central portion to a surface portion of thesilicon-based composite.

According to still another embodiment of the present specification, adiameter of the mesopore may be in a range of 2 to 50 nm, and a diameterof the macropore may be in a range of 50 to 500 nm.

According to yet another embodiment of the present specification, acrystal size of a crystalline portion of the silicon may be in a rangeof 1 to 50 nm.

According to yet another embodiment of the present specification, athickness of the carbon coating layer may be in a range of 0.003 to 3.0μm.

According to yet another embodiment of the present specification, aspecific surface area of the negative electrode active material may bein a range of 1 to 20 m²/g.

According to yet another embodiment of the present specification, aporosity of the negative electrode active material may be in a range of10 to 50%.

According to yet another embodiment of the present specification, anaverage particle diameter of the negative electrode active material maybe in a range of 0.1 to 20 μm, and an average particle diameter of thenegative electrode active material may be in a range of 0.5 to 10 μm.

In order to accomplish the object, yet another embodiment of the presentspecification provides a method of preparing a negative electrode activematerial, which includes: forming a carbon coating layer on asilicon-based precursor represented by SiO_(x) (0<x≤2); thermallytreating the silicon-based precursor on which the carbon coating layeris formed; and preparing a silicon-based composite represented bySiO_(a) (0≤a<1) and having a surface on which a carbon coating layer isdistributed by removing impurities, wherein the silicon-based compositehas a bimodal pore structure including mesopores and macropores.

According to yet another embodiment of the present specification, thecarbon coating layer may include one or more selected from the groupconsisting of natural graphite, artificial graphite, mesocarbonmicrobeads (MCMB), carbon fibers and carbon black.

According to yet another embodiment of the present specification, acontent of the carbon coating layer may be in a range of 1 to 50 wt % ofa total weight of the negative electrode active material.

According to yet another embodiment of the present specification, thethermal treatment may include thermally reducing a silicon-basedprecursor with a metal reducing agent in an inert atmosphere.

According to yet another embodiment of the present specification, thethermal treatment may be performed in a temperature range of 650 to 900°C.

According to yet another embodiment of the present specification, thethermal treatment may be performed in a rotary kiln.

According to yet another embodiment of the present specification, themetal reducing agent may include one selected from the group consistingof Ti, Al, Mg, Ca, Be, Sr, Ba and a combination thereof.

According to yet another embodiment of the present specification, amolar ratio of the silicon-based precursor to the metal reducing agentmay be in a range of 1:0.001 to 1:1.

According to yet another embodiment of the present specification, thepreparing the silicon-based composite may include removing impuritiesusing an acidic aqueous solution

According to yet another embodiment of the present specification, theacidic aqueous solution may include one or more selected from the groupconsisting of hydrochloric acid, nitric acid and sulfuric acid.

According to yet another embodiment of the present specification, theimpurities may include one or more materials selected from the groupconsisting of a metal oxide, a metal silicide and a metal silicate, andthe metal is one selected from the group consisting of Ti, Al, Mg, Ca,Be, Sr, Ba and a combination thereof.

In order to accomplish the object, yet another embodiment of the presentspecification provides a negative electrode for a lithium secondarybattery including the above-described negative electrode activematerial.

In order to accomplish the object, yet another of the presentspecification provides a lithium secondary battery including theabove-described negative electrode.

Advantageous Effects

A negative electrode active material according to the presentspecification includes silicon (Si) and a silicon-based composite havinga low oxygen content, and thus, when the negative electrode activematerial is applied to a lithium secondary battery, initial capacity andefficiency can be improved.

Further, according to the negative electrode active material of thepresent specification, since a silicon-based precursor is reduced afterthe carbon coating layer is formed, the crystal growth of silicon can beeasily controlled, reduction can be uniformly performed inside thereof,a pore structure is well developed throughout the particle, and thus alithium secondary battery with lifetime characteristics improved due toenhancement of swelling characteristics can be provided.

Further, according to the negative electrode active material of thepresent specification, since the specific surface area of the negativeelectrode active material is reduced due to the carbon coating layer,the side reaction with an electrolyte can be reduced.

DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron microscope (SEM) image of a cross-section(inside) of a silicon-based composite of Example 1 according to thepresent specification.

FIG. 2 is an SEM image of a surface of the silicon-based composite ofExample 1 according to the present specification.

FIG. 3 is an SEM image of a cross-section (inside) of the silicon-basedcomposite of Comparative Example 1.

FIG. 4 is an SEM image of a surface of the silicon-based composite ofComparative Example 1.

FIG. 5 is an SEM image of a cross-section (inside) of the silicon-basedcomposite of Comparative Example 2.

FIG. 6 is an SEM image of a surface of the silicon-based composite ofComparative Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLES

Hereinafter, the present specification will be described in more detail.However, the following examples are merely provided to allow for aclearer understanding of the present specification, rather than to limitthe scope thereof.

Example 1 1. Preparation of SiO Having Carbon Coating Layer FormedThereon

100 g of SiO was introduced into a rotary kiln, argon gas was flowedthereinto at a rate of 0.5 L/min and a temperature was raised up to1000° C. at a rate of 5° C./min. Thermal treatment was performed for 5hours while rotating the rotary kiln at a rate of 30 rpm/min, flowingargon gas at a rate of 1.8 L/min and flowing acetylene gas at a rate of0.5 L/min, and thereby SiO having a carbon coating layer formed thereonwas prepared. Here, a carbon content of the carbon coating layer was 10wt % based on SiO.

2. Preparation of Silicon-Based Composite

100 g of the prepared SiO, (x=1) having a carbon coating layer formedthereon and 41 g of Mg powder as a metal reducing agent were mixed, andthen put in a reaction vessel of a thermal reduction chamber.Subsequently, the temperature of the chamber was increased to 750° C.Thereafter, Ar was used as an inert gas, and Ar was supplied at a flowrate of about 800 sccm.

Further, the reaction was performed using a rotary kiln as the reactionvessel.

The thermal reduction reaction was performed for 12 hours, and thechamber temperature was decreased to room temperature after 12 hours. Aproduct in the reaction vessel was collected to prepare a silicon-basedcomposite.

Reduced MgO or the like was removed from the prepared silicon-basedcomposite using HCl (1N).

3. Preparation of Negative Electrode

The silicon-based composite with a carbon coating layer formed thereonprepared in the above 2 as a negative electrode active material,acetylene black as a conductive agent, and polyvinylidene fluoride as abinder were mixed at a weight ratio of 95:1:4 and the mixture was mixedwith N-methyl-2-pyrrolidone as a solvent to prepare a slurry. Onesurface of a copper current collector was coated with the preparedslurry to have a thickness of 30 μm, dried, rolled and punched into apredetermined size to prepare a negative electrode.

4. Preparation of Lithium Secondary Battery

A coin-type half cell (2016 R-type half cell) was prepared using thenegative electrode, a lithium counter electrode, a microporouspolyethylene separator and an electrolyte in a helium-filled glove box.A solution prepared by dissolving 1 M LiPF₆ in a solvent in whichethylene carbonate and dimethyl carbonate were mixed in a volume ratioof 50:50 was used as the electrolyte.

Comparative Example 1 1. Preparation of Silicon-Based Composite

A silicon-based composite including silicon with a porous structure wasprepared using an etching process.

2. Preparation of Secondary Battery

A lithium secondary battery was prepared in the same manner as inExample 1 except that the silicon-based composite prepared in the above1 was used as a negative electrode active material.

Comparative Example 2 1. Preparation of Silicon-Based Composite

A silicon-based composite was prepared in the same manner as in Example1 except that no carbon coating layer was formed.

2. Preparation of Secondary Battery

A lithium secondary battery was prepared in the same manner as inExample 1 except that the silicon-based composite prepared in the above1 was used as a negative electrode active material.

Experimental Example 1 Observation of Morphology

In order to determine the morphology of the surface and inside of thesilicon-based composites prepared in Example 1 and Comparative Examples1 and 2, the silicon-based composites were observed using a scanningelectron microscope, and the results are shown in FIGS. 1 to 6.

Referring to FIGS. 2, 4 and 6 showing the surface of the silicon-basedcomposites of Example 1 and Comparative Examples 1 and 2, it can be seenthat there is a large difference in the shape of the outer surface. Inthe case of FIG. 2 showing the surface of Example 1, almost no pore isobserved on the surface due to the carbon coating layer. However, in thecase of FIGS. 4 and 6 showing the surfaces of Comparative Examples 1 and2, it is determined that there are quite a few pores on the outermostsurface.

Accordingly, it can be seen with the naked eye that the case ofComparative Example 1 which is porous silicon of which the specificsurface area of the outermost surface as a portion in contact with anelectrolyte was formed by an etching process and the case of ComparativeExample 2 in which the carbon coating layer was not formed havesignificantly higher values than those of Example 1 which is a poroussilicon-based composite prepared by the method according to the presentspecification, and thus it can be confirmed that the side reaction withthe electrolyte can be reduced in the case of Example 1.

Further, referring to FIGS. 1, 3 and 5 in which the internal structureis observed, it can be confirmed that pores are hardly distributed inthe interior of Comparative Example 1 of FIG. 3, and in the case ofComparative Example 2 of FIG. 5, the pore distribution of the inner sideis not uniform as compared with the surface portion. On the other hand,in the case of Example 1 of FIG. 1, it was confirmed that the portionclose to the central portion or the surface or the whole portion had auniform pore structure.

Accordingly, it was found that the swelling phenomenon can be greatlyreduced due to the uniform pore structure formed inside, the lifetimecharacteristics of the battery can be improved due to the reduction involume expansion, and initial capacity and efficiency can also beincreased.

Experimental Example 2: Performance Evaluation of Secondary Battery 1.Measurement of Initial Discharge Capacity, Initial Efficiency andLifetime Characteristics

In order to determine initial discharge capacities of the coin-type halfcells prepared in Example 1 and Comparative Examples 1 and 2, thecoin-type half cells were charged and discharged to 0.1 C at a voltageof 0 V to 1.5 V once, and initial discharge capacities, initial chargecapacities and Coulombic efficiency were measured.

The measurement results of the initial discharge capacity, initialefficiency and lifetime characteristics measured by the above-describedmethod are shown in the following Table 1.

2. Measurement of Lifetime Characteristics and Change in Thickness(Swelling)

After the initial charging and discharging were performed on thecoin-type half cells prepared in Example 1 and Comparative Examples 1and 2, charging and discharging were carried out 49 times at a 0.5C-rate in the same voltage range, the difference of the initialthickness and the final thickness after the last charging anddischarging were measured and a thickness increase rate is shown in thefollowing Table 1.

TABLE 1 Initial discharge Initial Lifetime capacity efficiencycharacteristics Swelling (mAh/g) (%) (%) (%) Example 1 3100 90 60 190Comparative 3050 88 30 280 Example 1 Comparative 3120 84 35 260 Example2

Initial efficiency (%): (Discharge capacity of first cycle/Chargecapacity of first cycle)×100

Lifetime characteristics: (Discharge capacity of 49_(th) cycle/Dischargecapacity of first cycle)×100

Swelling (%): {(Final thickness−initial thickness)Initial thickness}×100

As shown in Table 1, as a result of measuring and comparing the initialdischarge capacity and swelling of Examples 1 and Comparative Examples 1and 2, the initial efficiency of the secondary batteries of Examples 1including the silicon-based composite having the carbon coating layerformed thereon increased by about 2 to 6% and the lifetimecharacteristics thereof was improved by about 25 to 30% as compared tothe secondary battery of Comparative Example 1 including thesilicon-based composite formed by an etching process and the secondarybattery of Comparative Example 2 including the silicon-based compositeon which the carbon coating layer was not formed. On the other hand, theswelling phenomenon was reduced by 70 to 90%, from which it can bedetermined that the safety of the battery is improved.

It can be seen that the efficiency of the battery is improved by beingimparted with the conductivity of the silicon-based composite, which isimparted by the carbon coating layer as in Example 1. Further, inExample 1, it was confirmed from the fact that lifetime characteristicsincreased and swelling phenomenon decreased that the reduction rate canbe controlled by reducing SiO after the formation of the carbon coatinglayer, and thus the inside of SiO can be uniformly reduced, andcrystalline Si and crystalline SiO₂ can be easily controlled.

While the present specification has been particularly shown anddescribed with reference to preferred embodiments thereof, it will beunderstood by those skilled in the art that various modifications andvariations may be made therein without departing from the spirit andscope of the invention as defined by the appended claims. Thus, theinvention is intended to cover all modifications, equivalents, andalternatives falling within the scope of the invention as defined by theappended claims.

Modes of the Invention

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Priorto the description, it should be understood that the terms or words usedin the specification and the appended claims should not be construed aslimited to general and dictionary meanings, but should be interpretedbased on the meanings and concepts corresponding to technical aspects ofthe present disclosure on the basis of the principle that the inventoris allowed to define terms appropriately for the best explanation.Therefore, the embodiments described in the specification and elementsillustrated in the drawings are just a preferable example for thepurpose of illustration only, not intended to limit the scope of thedisclosure, so it should be understood that other equivalents andmodifications could be made thereto at the time of filing theapplication.

An embodiment of the present specification provides a negative electrodeactive material which includes a silicon-based composite represented bySiO_(a) (0≤a<1), and a carbon coating layer distributed on a surface ofthe silicon-based composite, wherein the silicon-based composite has abimodal pore structure including mesopores and macropores.

Generally, in the silicon-based material, cracking of particles,chemical pulverization or the like easily occurs due to large changes involume (swelling) during charge and discharge, and thus there is aproblem in that lifetime characteristics rapidly decrease.

In order to address the above-described issue, an attempt to suppressthe volume expansion by preparing porous silicon using an etchingprocess at a surface portion has been made, but it was impossible tocompletely suppress the volume expansion using this method. Also, whenthe specific surface area becomes too large due to excessive etching, aside reaction with the electrolyte easily occurs.

However, the negative electrode active material according to the presentspecification includes a silicon-based composite represented by SiO_(a)(0≤a<1), and the silicon-based composite has a bimodal pore structureincluding mesopores and macropores. Therefore, in the lithium secondarybattery to which the negative electrode active material according to thepresent specification is applied, a volume expansion problem can beprevented to improve lifetime characteristics, initial efficiency andcapacity characteristics can be enhanced by controlling an oxygencontent of the product, and the specific surface area can be controlledas compared with other pore structures due to the carbon coating layer,such that the side reaction with the electrolyte can be reduced.

Further, although the total specific surface area may not besignificantly different from that of the negative electrode activematerial prepared using a conventional method, the pores of the bimodalstructure are uniformly distributed throughout from the inner centralportion to the surface portion of the composite, thereby contributing tothe performance improvement of the secondary battery as described above.

The negative electrode active material according to the presentspecification includes a silicon-based composite and a carbon coatinglayer, the silicon-based composite is represented by SiO_(a) (0≤a<1),the carbon coating layer may be in the form of a layer formed tosurround the surface of the silicon-based composite, and thesilicon-based composite has a bimodal pore structure including mesoporesand macropores.

When the silicon-based composite is used as a negative electrode activematerial, the silicon included in the silicon-based composite maysubstantially cause an electrochemical reaction when lithium ionsdeintercalated from a positive electrode active material are absorbed,stored and released. The crystalline characteristic of the silicon maypartially be amorphous, but mostly crystalline. This is because, whensilica is reduced by thermal reduction using a metallic gas to bedescribed below, the reduction is performed under the carbon coatinglayer at a relatively high temperature, and thus most of the silica maybe reduced to crystalline silicon.

A crystal size of crystalline silicon which is present in thesilicon-based composite may be 1 to 50 nm, and preferably 1 to 20 nm.Here, the crystal size may be determined by X-ray diffraction (XRD)analysis or electron microscopy (SEM and TEM). Although to be describedbelow, the reason why the crystalline size of the crystalline siliconmay be in the above-described range is that the thermal reduction can beperformed even at a low temperature, and uniform reduction of thesilicon-based composite as a whole can be achieved by forming the carboncoating layer even when a metal reducing agent having a strong reducingproperty is used, and a content of oxygen and a porous structure can beproperly controlled.

The silicon-based composite may be represented by SiO_(a) (0≤a<1). Inthe silicon-based composite, the ratio of crystalline silicon andcrystalline silica contained therein may be expressed as 1−(a/2):(a/2).In this case, the entire composition of the silicon-based composite maybe represented by SiO_(a) (0≤a<1).

Specifically, when a is 1 or more, the proportion of silica is higherthan that of silicon (the content of oxygen is higher) as compared tothe case in which a is less than 1, and the swelling phenomenon of thenegative electrode active material may be lowered to a certain extent,but the initial discharge capacity of the lithium secondary battery maybe reduced.

However, the silicon-based composite according to the presentspecification ultimately aims for the case where a is 0, and in thiscase, swelling properties can be greatly improved to increase lifetimecharacteristics. However, the present specification is not limited tothe silicon-based composite including only crystalline silicon. Since asilicon-based precursor remaining unreduced, or a silicon-basedprecursor produced by reoxidation may be present during the reductionreaction, a may have a range of 0 or more to less than 1.

The silicon-based composite may have a bimodal pore structure includingmesopores and macropores. Although to be described below, the bimodalpore structure is a space originally occupied by a metal oxide with alarge crystal size which is generated when silica is reduced by a metalreducing agent and a metallic material is oxidized, and may be formed bythe removal of the metal oxide. Since thermal reduction is performedafter forming the carbon coating layer, the reaction rate can be easilycontrolled due to the carbon coating layer, and thus, theabove-described pore structure can be uniformly formed all the wayinside of the silicon-based composite and have a pore structureincluding nanopores and mesopores as a bimodal pore structure.

A diameter of the mesopore of the bimodal pore structure may be in therange of 2 to 50 nm, and a diameter of the macropore may be in the rangeof 50 to 700 μm.

When the silicon-based composite has such a bimodal pore structure andnanopores and mesopores have a size within the above range, the porositycan be controlled by adjusting the amount of silicon to be reduced(i.e., the amount of the metal to be oxidized), and thereby the volumeexpansion thereof can be reduced, and the lifetime characteristics canbe improved. Further, a porosity of the silicon-based composite havingthe bimodal pore structure may be in the range of 10 to 50%, which maybe a range that can be controlled by the metal reducing agent.

The negative electrode active material according to the presentspecification may include a carbon coating layer distributed on asurface of the silicon-based composite. When the above-described carboncoating layer is formed, the reaction rate can be controlled and thereducing agent can be prevented from reacting only on the surface of thecrystalline silica during the reduction of the crystalline silica.Accordingly, the inside of the crystalline silica can be uniformlyreduced, and the oxygen content and the specific surface area can beeasily controlled.

When the carbon coating layer is formed on the surface of thesilicon-based composite, electrical conductivity is imparted to thesilicon-based composite, thereby improving the initial efficiency,lifetime characteristics, and battery capacity characteristics of thesecondary battery including the silicon-based composite.

The thickness of the carbon coating layer may be in the range of 0.003μm to 3.0 μm. When the thickness of the carbon coating layer is lessthan 0.003 μm, a carbon coating layer is too thin to contribute greatlyto improvement of electrical conductivity. When the thickness of thecarbon coating layer is more than 3.0 μm, the size of the negativeelectrode active material may become excessively large due to anexcessively thick carbon coating layer, the absorption, storing andrelease of lithium ions may be inhibited, and capacity and initialefficiency may be rather reduced.

The negative electrode active material including a silicon-basedcomposite having a carbon coating layer formed thereon according to anembodiment of the present specification may have a specific surface areain the range of 1 to 20 m²/g, which represents an area of a portion thatmay come into contact with an electrolyte, and may be less related toporosity due to a bimodal pore structure in the silicon-based composite.When the negative electrode active material has a specific surface areawithin the above range, the side reaction with the electrolyte can begreatly reduced.

Further, an average particle diameter of the negative electrode activematerial may be in the range of 0.1 to 20 μm, and preferably in therange of 0.5 to 10 μm. When the particle diameter of the negativeelectrode active material is less than 0.1 μm, an electrode density maybe reduced. When the particle diameter of the negative electrode activematerial is more than 20 μm, rate-determining characteristics may belowered, or lifetime characteristics may be reduced due to volumeexpansion.

Further, silicon particles used as a negative electrode active materialgenerally involve a very complicated crystal change in the reaction ofelectrochemical absorption, storing and release of lithium atoms. As thereaction of electrochemical absorption, storing and release of lithiumatoms proceeds, the composition and crystal structure of siliconparticles are changed to Si (crystal structure: Fd3m), LiSi (crystalstructure: I41/a), Li₂Si (crystal structure: C2/m), Li₇Si₂ (Pbam),Li₂₂Si₅ (F23), etc. Further, the volume of the silicon particles expandsby about four times as the complex crystal structure changes, and thereaction between the silicon-based composite according to an embodimentof the present specification and lithium atoms has an advantage in thatthe reaction can proceed while maintaining the crystal structure of thesilicon-based composite.

Another embodiment of the present specification provides a method ofpreparing the above-described negative electrode active material.

The method of preparing a negative electrode active material includesforming a carbon coating layer on a silicon-based precursor representedby SiO_(x) (0<x≤2); thermally treating the silicon-based precursor onwhich the carbon coating layer is formed; and preparing a silicon-basedcomposite represented by SiO_(a) (0≤a<1) having a surface on which acarbon coating layer is distributed by removing impurities.

According to an embodiment of the present specification, the formationof a carbon coating layer on a silicon-based precursor may be theformation of a coating layer by covering a surface of the silicon-basedprecursor with a carbon-based material before the silicon-basedprecursor which is a raw material is reduced.

The silicon-based precursor represented by SiO_(x) (0<x≤2), which is araw material, may include a crystalline silica, a material in whichcrystalline silicon and crystalline silica form a composite and amixture including amorphous material. Also a material in which the twomaterials among these are mixed may be applied as a raw material.

Further, a carbon coating layer formed on a surface of the silicon-basedprecursor as a coating layer may include, for example, graphite such asnatural graphite and artificial graphite, carbon fibers such asmesocarbon microbeads (MCMB), carbon nanotubes and carbon nanofibers,carbon black such as Ketjen black, Denka black and acetylene black, or amixture thereof, and any carbon source enabling carbon coating on thesurface of the silicon-based precursor may be applied without particularlimitation.

The formation of the carbon coating layer as described above may beperformed by dispersing the carbon precursor in a solvent such astetrahydrofuran (THF), an alcohol or the like, and adding a mixture thusobtained to the silicon-based precursor, followed by drying and thermaltreatment, and may be performed by supplying acetylene gas, but anycarbon-coating method typically used in the related field may be usedwithout particular limitation.

The content of the carbon coating layer may be in the range of 1 to 50wt % of the total weight of the negative electrode active material. Whenthe carbon coating layer is coated in an amount of less than 1 wt %, auniform coating layer may not be formed and conductivity may bedeteriorated. When the carbon coating layer is applied in an amount ofmore than 50 wt %, a coating layer becomes too thick, the size of thenegative electrode active material may become too large, and thecapacity and initial efficiency may be rather reduced. When the coatingamount is appropriately controlled, as described above, thesilicon-based composite is suitably imparted with electricalconductivity, and thereby the initial efficiency, lifetimecharacteristics, and battery capacity characteristics of the secondarybattery including the silicon-based composite can be improved.

In the method of preparing a negative electrode active materialaccording to an embodiment of the present specification, the reactionrate may be controlled because the silicon-based precursor is reducedafter the carbon coating layer is formed, and a reducing agent may beprevented from reacting only on a surface of the silicon-basedprecursor. Accordingly, the inside of the silicon-based precursor can beuniformly reduced, and thus porous silicon-based composites, ideallyporous silicon, can be prepared, the oxygen content in the producedsilicon-based composite can be easily controlled, and the carbon coatinglayer can also serve to reduce the specific surface area of the entirenegative electrode active material by preventing the crystal growth ofsilicon to a certain extent and maintaining the silicon after thereaction as a barrier layer, and basically serve to impart conductivityto the silicon-based active material.

According to an embodiment of the present specification, the step ofthermal treatment may be a step of reducing the silicon-based precursorhaving a surface coated with the carbon coating layer by heating underspecific conditions, and specifically, the step of thermal treatment mayinclude a step of thermally reducing the silicon-based precursor using ametal reducing agent under an inert atmosphere.

The thermal reduction of the silicon-based precursor may be performed bya process of thermally reducing the silicon-based precursor using ametallic powder or a metallic gas containing a metal reducing agent inan inert atmosphere. Oxygen is locally released in the form of a metaloxide by the metal in the silicon-based precursor by the thermalreduction, and thereby local reduction occurs.

That is, the content of oxygen decreases or only a very small amountremains as the silicon-based precursor is reduced, and as a result, asilicon-based composite including only crystalline silicon can beideally produced, or a silicon-based composite in which crystallinesilicon, (crystalline or amorphous) silica remaining unreduced, and(crystalline or amorphous) silica produced by reoxidation are present toform a composite can be generally produced. As described above, thesilicon prepared may be crystalline silicon, amorphous silicon, or amixture thereof

For example, Ti, Al, Mg, Ca, Be, Sr, Ba or a combination thereof may beapplied as the metal reducing agent, and thermal reduction may beperformed using powders or gases of the above-described metals. Any typeof a metal reducing agent may be used without limitation as long as ithas sufficient reducing power to separate or extract oxygen from theabove-described silicon-based precursor described above, and preferably,magnesium (Mg) may be used.

Further, the thermal treatment may be performed at a temperature of 500°C. or higher, and preferably in a temperature range of 650 to 900° C.When the temperature of the thermal treatment is less than 500° C., itmay be difficult for the reduction reaction to occur due to a lowtemperature and a silicon-based composite with a high oxygen content maybe formed. When the temperature of the thermal treatment is more than900° C. or 1000° C., the crystal of silicon may grow large, and thecrystal characteristic may be degraded.

As described above, when the thermal reduction is carried out byperforming the thermal treatment at a temperature of 500° C. or more,strong reduction by a metal reducing agent may be achieved, the amountof oxygen in the silicon-based composite prepared by the reduction whichproceeds with diffusion can be easily controlled, and a silicon-basedcomposite having a bimodal pore structure, that is, SiO_(a) (0≤a<1) canbe prepared.

Further, a may be 0 in SiO_(a) which is the silicon-based composite, andthis is the most optimal form that the silicon-based composite can haveas described above, and may represent a silicon-based composite in whichonly porous silicon having a bimodal pore structure remains.

In this case, the initial efficiency and capacity characteristics may beremarkably excellent. When the oxygen content is low, although problemsdue to volume expansion may occur, the silicon-based composite isporous, and thus the problems due to volume expansion may also beprevented, and thereby a negative electrode active material havingexcellent properties can be provided.

However, even when no reduction reaction occurs, or oxygen is stillpresent in the composite due to reoxidation and a is not 0 but has avalue in the range of 0 to 1, the lifetime characteristics can begreatly improved as compared with the case where the porous silicon isconventionally formed through the etching process, and the specificsurface area which is a contact area with the electrolyte is reduced dueto the formation of the carbon coating layer to greatly contribute toimproving the lifetime characteristics and conductivity.

Further, the thermal reduction may be performed while flowing an inertgas, and examples of the inert gas which may be used herein include, forexample, Ar, N₂, Ne, He, Kr, or a mixed gas thereof.

The step of thermal treatment may include performing a reaction of amixture obtained by mixing a metallic powder or a metallic gascontaining a metal reducing agent such as magnesium with thesilicon-based precursor in a reaction furnace. For example, the reactionmay be performed in a rotary kiln to maintain a uniform reaction bymaximizing a contact area between the silicon-based precursor and Mgwhich is a metal reducing agent.

In the method of preparing a negative electrode active materialaccording to the present specification, the amount of oxygen in thefinally prepared silicon-based composite may be controlled by adjustingthe ratio of the silicon-based precursor and the metal reducing agent inthe step of thermal treatment. In order to control the amount of oxygenin the silicon-based composite, a molar ratio of the silicon-basedprecursor to a metal reducing agent may be in the range of 1:0.001 to1:1.

As the amount of the metal reducing agent becomes larger, a largeramount of the silicon-based precursor may be reduced, and thus theamount of oxygen contained in the prepared silicon-based composite canbe easily controlled by controlling the amount of the metal reducingagent used in the thermal reduction, and the ratio of silicon in thesilicon-based composite may be further increased.

As an example, Mg may be included as the metal reducing agent.

Accordingly, a stoichiometric reaction of the silicon-based precursorand Mg as the reducing agent is as follows:

2Mg+SiO₂→Si+2MgO   Reaction Formula 1

That is, as shown in Reaction Formula 1, the metal, as a reducing agent,reduces silica, whereby the metal may be oxidized to produce a metaloxide, and the silica may be reduced to produce silicon. In addition, ametallic reducing agent other than Mg may be used as the reducing agent,and in this case, the reduction of the silicon-based precursor occurs bya reaction similar to the above reaction formula.

According to an embodiment of the present specification, the preparingof the silicon-based composite may include removing impurities using anacidic aqueous solution.

As the acidic aqueous solution, for example, hydrochloric acid, nitricacid, sulfuric acid and the like may be used, and preferably, an aqueoushydrochloric acid solution may be used and may be used at aconcentration in the range of about 0.1 to 10 N. When hydrochloric acidis used at a concentration of less than 0.1 N, impurities may not becompletely removed. When hydrochloric acid is used at a concentration ofmore than 10 N, preparation efficiency may be lowered. Examples of theremoved impurities include MgO, Mg₂Si, Mg₂SiO₄, etc., and the impuritiesmay vary depending on the type of metal used as a metal reducing agent.

After impurities including metal oxides are removed from thesilicon-based composite, a silicon-based composite including amorphoussilicon, crystalline silicon and crystalline silica may be obtainedafter undergoing general cleaning and drying processes.

As described above, the silicon-based composite prepared by reducing SiOmay include crystalline silicon, amorphous silicon and crystallinesilica. The negative electrode active material including thesilicon-based composite can allow the reaction between the amorphoussilica and lithium contained in the electrolyte to be excluded, andimprove the initial efficiency and capacity characteristics of thesecondary battery.

Still another embodiment of the present specification provides a lithiumsecondary battery including the negative electrode active materialprepared by the method of preparing the negative electrode activematerial.

The lithium secondary battery includes a positive electrode including apositive electrode active material; a separator; a negative electrodeincluding the negative electrode active material; and an electrolyte,and the negative electrode may be prepared with the negative electrodeactive material. For example, the negative electrode active materialaccording to an embodiment of the present specification is mixed with abinder, a solvent, and a conductive agent and a dispersant as necessary,and stirred to prepare a slurry. Then, a current collector may be coatedwith the slurry and pressed to prepare the negative electrode.

Examples of the binder include a polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol,carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose,regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, polyacrylate, an ethylene-propylene-dienemonomer (EPDM), a sulfonated EPDM, styrene-butadiene rubber (SBR),fluoro rubber, various copolymers, etc.

Examples of the solvent include N-methyl-2-pyrrolidone, acetone, waterand the like.

The conductive agent is not particularly limited as long as it hasconductivity and does not generate chemical changes in the battery.Examples thereof include graphite such as natural graphite andartificial graphite; carbon black such as acetylene black, Ketjen black,channel black, furnace black, lamp black, and thermal black; conductivefibers such as carbon fibers and metal fibers; metal powder such asfluorocarbon powder, aluminum powder, and nickel powder; conductivewhiskers such as zinc oxide whiskers and potassium titanate whiskers; aconductive metal oxide such as titanium oxide; a conductive materialsuch as a polyphenylene derivative, etc.

An aqueous-based dispersant or an organic dispersant such asN-methyl-2-pyrrolidone may be used as the dispersant.

As in the preparation of the negative electrode, a positive electrodeactive material, a conductive agent, a binder, and a solvent are mixedto prepare a slurry, and then a positive electrode may be prepared bydirectly coating a metal current collector with the slurry or by castingthe slurry on a separate support and laminating a positive electrodeactive material film separated from the support on a metal currentcollector.

Examples of the positive electrode active material may be a layeredcompound, such as lithium cobalt oxide (LiCoO₂) or lithium nickel oxide(LiNiO₂), or a compound substituted with at least one transition metal;lithium manganese oxides represented by the chemical formulaLi_(i+y)Mn_(2−y)O₄ (where y is in the range of 0 to 0.33), LiMnO₃,LiMn₂O₃, and LiMnO₂; lithium copper oxide (Li₂CuO₂); vanadium oxidessuch as LiV₃O₈, LiFe₃O₄, V₂O₅, and Cu₂V₂O₇; nickel (Ni)-site typelithium nickel oxide represented by the chemical formula ofLiNi_(1−y)M_(y)O₂ (where M is cobalt (Co), manganese (Mn), aluminum(Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), or gallium(Ga), and y is in the range of 0.01 to 0.3); a lithium manganesecomposite oxide represented by the chemical formula of LiMn_(2−y)M_(y)O₂(where M is cobalt (Co), nickel (Ni), iron (Fe), chromium (Cr), zinc(Zn), or tantalum (Ta), and y is in the range of 0.01 to 0.1) orLi₂Mn₃MO₈ (where M is Fe, Co, Ni, Cu, or Zn); and LiMn₂O₄ having a partof lithium (Li) being substituted with alkaline earth metal ions, butthe present specification is not limited thereto.

As the separator, a typical porous polymer film used as a conventionalseparator, for example, a porous polymer film prepared using apolyolefin-based polymer, such as an ethylene homopolymer, a propylenehomopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer,and an ethylene/methacrylate copolymer, may be used alone or by beinglaminated. A typical porous nonwoven fabric, for example, a nonwovenfabric formed of high melting point glass fibers or polyethyleneterephthalate fibers may be used, but the present specification is notlimited thereto.

In an electrolyte used in an embodiment of the present specification, alithium salt, which may be included as the electrolyte, may be usedwithout limitation as long as it is commonly used in an electrolyte fora secondary battery. An example of an anion of the lithium salt includesone selected from the group consisting of F⁻, Cl⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻,BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻,(CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻,CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻,CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and (CF₃CF₂SO₂)₂N—.

In the electrolyte used in the present specification, an organic solventincluded in the electrolyte may be used without limitation as long as itis commonly used, and one or more selected from the group consisting ofpropylene carbonate, ethylene carbonate, diethyl carbonate, dimethylcarbonate, ethylmethyl carbonate, methylpropyl carbonate, dipropylcarbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane,diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone,propylene sulfite, and tetrahydrofuran may be typically used.

Particularly, ethylene carbonate and propylene carbonate, which arering-type carbonates among the carbonate-based organic solvents,dissociate the lithium salt in the electrolyte well due to highdielectric constants as organic solvents with high viscosity, and thusmay be preferably used. An electrolyte having high electricalconductivity may be prepared when the ring-type carbonate is mixed witha linear carbonate having low viscosity and a low-dielectric constant,such as dimethyl carbonate and diethyl carbonate, in a suitable ratio,and thus the ring-type carbonate may be more preferably used.

Selectively, the electrolyte stored according to an embodiment of thepresent specification may further include additives such as anovercharge inhibitor contained in conventional electrolytes.

A separator is disposed between the positive electrode and the negativeelectrode to form a battery structure, the battery structure is wound orfolded to be placed in a cylindrical battery case or prismatic batterycase, and then a secondary battery is completed when the electrolyte isinjected thereinto. Also, the battery structure is stacked in a bi-cellstructure, impregnated with the electrolyte, and a secondary battery isthen completed when the product thus obtained is put in a pouch andsealed.

1. A negative electrode active material, comprising: a silicon-basedcomposite represented by SiO_(a) (0≤a<1); and a carbon coating layerdistributed on a surface of the silicon-based composite; wherein thesilicon-based composite has a bimodal pore structure including mesoporesand macropores.
 2. The negative electrode active material according toclaim 1, wherein the silicon-based composite has the bimodal porestructure formed entirely from an inner central portion to a surfaceportion of the silicon-based composite.
 3. The negative electrode activematerial according to claim 1, wherein a diameter of the mesopore is ina range of 2 to 50 nm.
 4. The negative electrode active materialaccording to claim 1, wherein a diameter of the macropore is in a rangeof 50 to 700 nm.
 5. The negative electrode active material according toclaim 1, wherein a thickness of the carbon coating layer is in a rangeof 0.003 to 3.0 μm.
 6. The negative electrode active material accordingto claim 1, wherein a specific surface area of the negative electrodeactive material is in a range of 1 to 20 m²/g. 7-8. (canceled)
 9. Amethod of preparing a negative electrode active material, comprising:forming a carbon coating layer on a silicon-based precursor representedby SiO_(x) (0<x≤2); thermally treating the silicon-based precursor onwhich the carbon coating layer is formed; and preparing a silicon-basedcomposite represented by SiO_(a) (0≤a<1) and having a surface on which acarbon coating layer is distributed by removing impurities, wherein thesilicon-based composite has a bimodal pore structure including mesoporesand macropores.
 10. (canceled)
 11. The method according to claim 9,wherein a content of the carbon coating layer is in a range of 1 to 50wt % of a total weight of the negative electrode active material. 12.The method according to claim 9, wherein the thermal treatment includesthermally reducing a silicon-based precursor with a metal reducing agentin an inert atmosphere.
 13. The method according to claim 9, wherein thethermal treatment is performed in a temperature range of 650 to 900° C.14. (canceled)
 15. The method according to claim 12, wherein the metalreducing agent includes one selected from the group consisting of Ti,Al, Mg, Ca, Be, Sr, Ba and a combination thereof
 16. The methodaccording to claim 12, wherein a molar ratio of the silicon-basedprecursor to the metal reducing agent is in a range of 1:0.001 to 1:1.17. The method according to claim 9, wherein the preparing of thesilicon-based composite includes removing impurities using an acidicaqueous solution.
 18. (canceled)
 19. The method according to claim 9,wherein the impurities include one or more materials selected from thegroup consisting of a metal oxide, a metal silicide and a metalsilicate, and the metal is one selected from the group consisting of Ti,Al, Mg, Ca, Be, Sr, Ba and a combination thereof.
 20. A secondarybattery comprising the negative active material of claim 1.